Engine lubricating oil to be used for a two-wheeled vehicle four-cycle engine differs from four-wheeled vehicle four-cycle engine lubricating oil, and must satisfy properties required for lubrication of both of an engine and a power transmission device, such as a transmission.
For many systems such as two-wheeled vehicles and the like having wet clutch and employing the same oil for engine lubricating oil and power transmission lubricating oil for lubrication, four-wheeled vehicle four-cycle engine lubricating oil is desirably not used as it is. Many improvement studies regarding an engine lubricating oil composition appropriate for a two-wheeled vehicle four-cycle engine are conducted (see Patent Documents 1 and 2, for example).
Vehicle engine lubricating oil must have various properties such as: (1) good detergency; (2) excellent anti-wear property; (3) excellent heat/oxidation stability; (4) small oil consumption; and (5) small engine frictional loss (excellent fuel saving property) in particular, a two-wheeled vehicle has higher engine revolution and higher power with respect to the displacement in normal use compared with those of a four-wheeled vehicle. Further, two-wheeled vehicle engine lubricating oil differs from four-wheeled vehicle engine lubricating oil and is used for lubrication of both of an engine and a power transmission device such as a transmission. Thus, the two-wheeled vehicle engine lubricating oil has an increased oil temperature and must have higher high-temperature detergency compared with that of four-wheeled vehicle engine lubricating oil.
Further, considering the recent environmental issues, in particular, reduction of carbon dioxide emissions, vehicle fuel saving has become an important issue. For vehicle fuel saving, engine lubricating oil requires a fuel saving technique to be applied. For improvement in fuel saving property of a power transmission device such as a transmission, improvement in a power transmission rate and reduction in size and weight are required. From the viewpoints of securing clutch capacity and reduction in weight of the clutch, a friction coefficient between a clutch disc and a clutch plate is required to increase. Thus, the two-wheeled vehicle four-cycle engine lubricating oil is desired to have properties required for engine system lubricating oil, and the power transmission device is desired to have further improved frictional property (high static friction coefficient) of wet clutch.
Patent Document 1: JP 2001-214184 A (page 2)
Patent Document 2: JP 2001-31984 A (page 2)